The term color space generally refers to a model, generally numerical in nature that describes colors in a particular environment, such as the colors capable of display by a particular device. There are color spaces that are based on real world tri-stimulus displays and there are color spaces that are designed based on the CIE Color Matching functions or other encoding rules in order to cover all perceptible colors rather than all displayable colors. However, if the latter color spaces are used, then a display must have a recipe to deal with all legal colors of the color space which is a far too complicated task for a consumer type of display. The solution is to put constraints on the unconstrained color spaces in order to allow for a realizable conversion of the transmitted colors to the display gamut.
The DCI (Digital Cinema Initiative) deliberately does not limit the color palette, nor does the DCI propose a particular gamut conversion method in order to allow for new kinds of footage and for new kinds of display devices (e.g., projectors). To understand the problem, consider the following situation depicted in FIG. 1. A camera 101 records a real life scene with its sensor that has three types of sensors, red, green and blue, generating CIE 1932 tristimulus values Xr,Yr,Zr for Red, Xg,Yg,Zg for Green, and Xb,Yb,Zb for Blue, respectively. Alternatively, the camera 101 could use also a different number of sensor types and the sensor sensitivities need not be described with “Red”, “Green”, “Blue”, but by “Blue-Cyan” and “Yellow” or the like, but the first definition encompasses the most common situation. In a second step, the signals from the sensors within the camera are converted to the XYZ color space or another color space which is greater than the color gamut of the camera 101. In a third step, this data is transmitted to a sink (display) device 102 using this XYZ color space. The sink device 102, in this example comprises consumer type display, and uses a gamut conversion method to convert from XYZ or another color gamut to its own color gamut.
A problem can arise when the sink device (or display device) 102 has have a color gamut (space) which is smaller, and indeed significantly smaller, than the color gamut which is described by another color space, like the XYZ color space, or the xvYCC color space. Thus, the display device will need a mechanism for finding a display color for each individual color that is possible within the color gamut (color space) specific to the display device. The need to display each color in a different color space can give rise to additional problems. First, many algorithms use the source/sink gamut differences to determine the gamut conversion methodologies (e.g., soft clipping etc.). In the case of color spaces like XYZ and xvYCC as defined in IEC 61966-2-3, a very large color gamut must to be covered, and only a fraction of the possible input colors are actually used with real sources.